Power steering system

ABSTRACT

An improvement in a known power steering system of the type in which movement of a steering wheel is transmitted via a torsion bar to an oil path switching valve to actuate a power cylinder in a desired steering direction by switching a high pressure oil path extending from an oil pump to the oil path switching valve and a low pressure oil path extending from the oil path switching valve to an oil tank. A part of the working oil flowing through the high pressure oil path is led to a reaction piston to restrain torsion of the torsion bar. The improvement is constituted by a cylindrical portion formed in the output shaft so as to surround the end portion on the output shaft side of the input shaft, a plurality of reaction pistons disposed within the cylindrical portion of the output shaft along their center lines lying on a plane perpendicular to the center axis of the input shaft and arrayed radially at an equal interval in a circumferential direction, a control oil path branched from the high pressure oil path and communicating with the reaction pistons, and a plurality of longitudinal grooves formed in the outer peripheral surface of the end portion on the output shaft side of the input shaft along the center axis of the input shaft and engaged with protrusions on the reaction pistons.

This application is a continuation of now abandoned application Ser. No.594,479 filed Mar. 27, 1984.

The present invention relates to improvements in a power steering systemof the type in which movement of a steering wheel is transmitted via atorsion bar to an oil path switching valve to actuate a power cylinderin a desired steering direction by switching a high pressure oil pathextending from an oil pump to the oil path switching valve and a lowpressure oil path extending from the oil path switching valve to an oiltank and a part of working oil flowing through the high pressure oilpath is led to a reaction piston to restrain torsion of the torsion bar.

Various designs of a power steering system of the above-mentioned typehave been heretofore known. However, some known power steering systemsof the above-mentioned type had a shortcoming that the reaction effectproduced by the reaction piston was not sufficient and it was difficultto arbitrarily preset a torsion torque of the reaction effect.

It is therefore one object of the present invention to provide animproved power steering system in which a large reaction effect can beobtained and a torsion torque of the reaction effect can be arbitrarilypreset.

According to one feature of the present invention, there is provided apower steering system comprising a cylindrical input shaft coupled to asteering wheel, a torsion bar extending through the inside of thecylindrical input shaft with one end fixedly secured to one end portionof the input shaft and the other end projected from the other open endportion of the input shaft for transmitting rotation of the input shaftto the output shaft, an oil path switching valve in which oil paths areswitched according to a rotational angle difference between the inputshaft and the output shaft, a power cylinder coupled to the outputshaft, a high pressure oil path for supplying working oil delivered froman oil pump to the power cylinder via the oil path switching valve, acylindrical portion formed in the output shaft so as to surround the endportion on the output shaft side of the input shaft, a plurality ofreaction pistons disposed within the cylindrical portion of the outputshaft along their center lines lying on a plane perpendicular to thecenter axis of the input shaft and arrayed radially at an equal intervalin a circumferential direction, a control oil path branched from thehigh pressure oil path and communicating with the reaction pistons, anda plurality of longitudinal grooves formed in the outer peripheralsurface of the end portion on the output shaft side of the input shaftalong the center axis of the input shaft and engaged with a protrusionof the reaction pistons.

The above-described and other features and objects of the presentinvention will become more apparent by reference to the followingdescription of preferred embodiments of the invention taken inconjunction with the accompanying drawings, wherein:

FIG. 1 is an oil hydraulic circuit diagram showing one preferredembodiment of a power steering system according to the presentinvention,

FIG 2. is a longitudinal cross-section of an oil path switching valve,

FIG. 3 is a transverse cross-section of a lower portion of the samevalve,

FIG. 4 is a transverse cross-section of an upper portion of the samevalve,

FIG. 5 is a longitudinal cross-section of a change-over valve, apressure control valve and a flow rate control valve,

FIG. 6 is another longitudinal cross-section of the oil path switchingvalve and the pressure control valve,

FIG. 7 is another longitudinal cross-section of the oil path switchingvalve and the change-over valve,

FIG. 8(I) is an enlarged longitudinal cross-section of the change-overvalve, the pressure control valve and the flow rate control valve,

FIG. 8 (II) is an end view of the flow rate control valve,

FIG. 9 is an enlarged longitudinal cross-section of the pressure controlvalve,

FIG. 10 is another enlarged longitudinal cross-section of the samevalve,

FIG. 11 is an enlarged plan view of the sleeve in the pressure controlvalve,

FIG. 12 is an enlarged longitudinal cross-section of the same sleeve,

FIG. 13 is another enlarged longitudinal cross-section of the samesleeve,

FIG. 14 is a transverse cross-section taken along line XIV--XIV in FIG.12 as viewed in the direction of the arrows,

FIG. 15 is a transverse cross-section taken along line XV--XV in FIG. 13as viewed in the direction of the arrows,

FIG. 16 is a transverse cross-section taken along line XVI--XVI in FIG.12 as viewed in the direction of the arrows,

FIG. 17 is a transverse cross-section taken along line XVII--XVII inFIG. 13 as viewed in the direction of the arrows,

FIG. 18 is a side view of the sleeve in the same pressure control valve,

FIG. 19 is a longitudinal cross-section showing the sleeve and a spoolin the same valve,

FIG. 20 is a side view showing the same spool,

FIG. 21 is an enlarged longitudinal cross-section of a sleeve and aspool in the flow rate control valve,

FIG. 22 is a transverse cross-section of a filter,

FIG. 23 is a front view of the same filter,

FIG. 24 is a transverse cross-section showing a mounted state of thesame filter,

FIG. 25 is a circuit diagram of a control apparatus,

FIG. 26 is a schematic diagram showing the relation between the outputoil pressure of an oil path switching valve (a delivery pressure of apump) and the torsion angle of a torsion bar (a relative angulardisplacement between a spool and an input shaft in an oil path switchingvalve),

FIG. 27 is a schematic diagram showing the relation between the outputoil pressure and the steering wheel torque,

FIG. 28 is a schematic diagram showing the relation between the oilpressure in a reaction plunger side chamber (a steering wheel torque)and the torsion angle of a torsion bar,

FIG. 29 is a schematic diagram showing the relation between the oilpressure in the reaction plunger side chamber and the output oilpressure,

FIG. 30 is a schematic diagram showing the relation between the steeringwheel torque and the torsion angle of the torsion bar,

FIG. 31 is a schematic diagram showing the flow rate on an inlet side ofa control system and flow rates at various portions within the controlsystem,

FIG. 32 is a plan view showing reaction pistons of the prior art,

FIG. 33 is a plan view showing reaction pistons according to the presentinvention,

FIG. 34 is a schematic view for explaining the operation of the reactionpistons in FIG. 33,

FIGS. 35 and 36 are schematic views for explaining the reason why atorsion torque can be arbitrarily preset according to the presentinvention,

FIGS. 37 and 38 are schematic views showing other preferred embodimentsof the grooves formed in the input shaft,

FIG. 39 is a schematic diagram showing characteristics of the embodimentshown in FIGS. 35 and 36,

FIG. 40 is a schematic diagram showing a characteristic of theembodiment shown in FIG. 37,

FIG. 41 is a schematic diagram showing a characteristic of theembodiment shown in FIG. 38, and

FIGS. 42 and 43 are schematic views showing still other preferredembodiments of the reaction pistons.

Now the present invention will be described in greater detail withreference to FIGS. 1 to 43 of the accompanying drawings. Explaining, atfirst, the outline of the power steering system according to the presentinvention with reference to FIG. 1, reference numeral (1) designates anoil pump driven by an engine (not shown), and this oil pump (1) is anoil pump whose flow rate is constant (about 7 l/min.) and whose deliverypressure is variable (5 kg/cm² -70 kg/cm²). In addition, referencenumeral (2) designates a four way type oil path switching valve (rotaryvalve), numeral (3) designates a steering power cylinder, numeral (4)designates an oil tank, numeral (5) designates a plurality of reactionpistons, numeral (6) designates chambers formed on the rear side of therespective reaction pistons, numeral (7a) designates a high pressure oilpath extending from the oil pump (1) to the oil path switching valve,numeral (8a) designates a low pressure oil path extending from the oilpath switching valve (2) to the oil tank (4), numerals (9a) and (10a)designates oil paths extending from the oil path switching valve (2) tothe steering power cylinder (3), reference character (a) designates amain orifice provided midway of the high pressure oil path, numeral (7b)designates a bypassing oil path connected to the high pressure oil paths(7a) on the upstream side and on the downstream side, respectively, ofthe main orifice (a), numeral (11) designates a change-over valve (COV)forming oil pressure increase means which is interposed in the midway ofthe same bypassing oil path (7b), numeral (12) designates a pressurecontrol valve connected to the oil path (7b) on the upstream side of thechange-over valve (11) via an oil path (7c), numeral (13) designates aflow rate control valve, numeral (7d) designates an oil path extendingfrom the pressure control valve (12), and a pair of parallel oil paths(7e) and (7e') branched from the oil path (7d) extend to theabove-mentioned flow rate control valve (13). In addition, referencenumeral (7d₁) designates an auxiliary pilot oil path extending frommidway of the oil path (7d) to the above-mentioned pressure controlvalve (12), numeral (7d₂) designates an oil path extending from midwayof the oil path (7d) to the chambers (6) on the rear side of theaforementioned reaction pistons (5), numeral (7d₃) designates an oilpath extending from midway of the above-mentioned oil path (7d) to thelow pressure oil path (8b), reference characters (b) and (c) designatesecond and fourth orifices, respectively, provided midway of theabove-mentioned oil path (7e ), numeral (7e₁) designates a COV pilot oilpath extending from the oil path (7e) between the orifices (b) and (c)to the above-mentioned change-over valve (11), reference character (e)designates a third orifice provided midway of the oil path (7d₃),numeral (7f) designates an oil path extending from the above-mentionedflow rate control valve (13) to the low pressure oil path (8b),reference character (d) designates a first orifice provided midway ofthe oil path (7f), numeral (7f₁) designates a main pilot oil pathextending from the oil path (7f) on the upstream side of the firstorifice (d) to the above-mentioned pressure control valve (12), numeral(14) designates a car speed sensor, numeral (15) designates a controlapparatus, numeral (16) designates an ignition switch, numeral (17)designates an ignition coil, and numerals (18a) and (18b) designatewirings extendin from the ignition coil (17) to an electromagnetic coil(solenoid) of the above-mentioned flow rate control valve (13), the carspeed sensor (14) is adapted to detect the car speed and transmit apulse signal produced as a result of the detection (a pulse signalcorresponding to car speed to the control apparatus (15), and thecontrol apparatus (15) is adapted to send a current corresponding to thepulse signal (a current corresponding to a car speed which varies from azero current (i=0) upon a predetermined high speed to a maximum current(i=1) upon stoppage) to an electromagnetic coil (solenoid) (57) of theflowrate control valve (13) and hold the plunger (52) and a spool (51)of the flow rate control valve (13) at a predetermined positioncorresponding to the aforementioned current value.

Next, the above-referred to oil path switching valve (2), change-overvalve (11), pressure control valve (12) and flow rate control valve (13)will be described in more detail with reference to FIGS. 2 to 21.Reference numeral (20) in FIGS. 2 to 7 designates a valve housing, andthe above-mentioned respective valves (2), (11), (12) and (13) areassembled within the same valve housing (20). Explaining, at first, theoil path switching valve (2) in detail with reference to FIG. 2,reference numeral (21) designates an input shaft which is manipulated bya steering wheel (not shown), numeral (23) in FIGS. 2 and 3 designates acylinder block forming an output shaft which is rotatably supportedwithin the valve housing (20) by means of upper and lower bearings,numeral (22) designates a torsion bar inserted within theabove-mentioned input shaft (21), which is fixedly secured at its top tothe input shaft (21) and at its bottom to the cylinder block (23), andowing to torsion of the torsion bar (22) the input shaft (21) and thecylinder block (23) can undergo a relative rotational movement through acertain angle. In addition, reference numeral (21a) designates aplurality of longitudinal grooves provided in the outer peripheralsurface of the lower portion of the input shaft (21), and in thecylinder block (23) are provided cylinders opposed to the respectivelongitudinal grooves (21a), the aforementioned reaction pistons (5) arefitted in the respective cylinders, and projections provided at the tipends of the respective reaction pistons (5) are engaged with thecorresponding longitudinal grooves (21a). The chambers (6) on the rearside of the respective reaction pistons (5) are formed between thecylinder block (23) and the valve housing (20) and communicated with anannular groove (6').

Reference numeral (23a) designates a pinion formed integrally with thecylinder block (23), numeral (24a) designates a rack which is meshedwith the pinion (23a) and in turn coupled to a piston rod of the powercylinder (3), numeral (24) designates a rack support, numeral (26)designates a cap, numeral (25) designates a spring interposed betweenthe cap (26) and the rack support (24), numeral (28) designates a sleeveof the oil path switching valve (2) fixed within the valve housing (20)right above the aforementioned cylinder block (23), numerals (28a),(28b) and (28c) designate oil paths provided on the outer peripheralsurface of the sleeve (28), numeral (27) designates a valve body fittedbetween the sleeve (28) and the input shaft (21), numeral (23b)designates a pin for connecting a bottom end portion of the valve body(27) with a top end portion of the cylinder block (23), and numerals(27a), (27b) and (27c) designate oil paths provided on the outerperipheral surface of the valve body (27).

In the above-described construction, when a steering wheel is placed ata neutral position, the high pressure oil path (7a) communicates with achamber (29) between the input shaft (21) and the torsion bar (22)through the oil path (27a) in the valve body (27) and the oil path (28a)in the sleeve (28), and hence working oil supplied from the oil pump (1)will circulate through the route consisting of the high pressure oilpath (7a)→the oil path (28a)→the oil path (27a)→the chamber (29) (theoil paths between the oil path (27a) and the chamber (29) being notshown)→the low pressure oil path (8a)→the oil tank (4)→the oil pump (1).If the input shaft (21) is rotated in the clockwise direction as viewedfrom the above relatively to the valve body (27) by rotating a steeringwheel in the right turn direction, then the high pressure oil path (7a)communicates with the oil path (9a) for the power cylinder (3) throughthe oil path (28a) of the sleeve (28), the oil paths (27a) and (27b) ofthe valve body (27) and the oil path (28b) of the sleeve (28), while thelow pressure oil path (8a) communicates with the oil path (10a) for thepower cylinder (3) through the chamber (29), the oil path (27c) of thevalve body (27) and the oil path (28c) of the sleeve (28), and hence theworking oil supplied from the oil pump (1) is passed through the routeconsisting of the high pressure oil path (7a)→the oil path (28a)→the oilpaths (27a) and (27b)→the oil path (28b)→the oil path (9a)→the left ofthe power cylinder (3), while the oil in the right chamber of the powercylinder (3) is returned through the route of the oil path (10a)→the oilpath (28c)→the oil path (27c)→the chamber (29)→the low pressure oil path(8a)→the oil tank (4), so that the piston rod of the power cylinder (3)moves rightwardly, and thus steering in the right turn direction can beeffected. On the other hand, if the input shaft (21) is rotated in thecounterclockwise direction as viewed from the above relatively to thevalve body (27) by rotating a steering wheel in the left turn direction,then the high pressure oil path (7a) communicates with the oil path(10a) for the power cylinder (3) through the oil path (28a) of thesleeve (28), the oil path (27c) of the valve body (27) and the oil path(28c) of the sleeve (28), while the low pressure oil path (8a)communicates with the oil path (9a) for the power cylinder (3) throughthe chamber (29), the oil path (27b) of the valve body (27) and the oilpath (28b) of the sleeve (28), and hence the working oil supplied fromthe oil pump (1) is passed through the route consisting of the highpressure oil path (7a)→the oil path (28a)→the oil path (27c)→the oilpath (28c)→the oil path (10a)→the right chamber of the power cylinder(3), while the oil in the left chamber of the power cylinder is returnedthrough the route of the oil path (9a)→the oil path (28b)→the oil path(27b)→the chamber (29)→the low pressure oil path (8a)→the oil tank (4),so that the piston rod of the power cylinder (3) moves leftwardly, andthus steering in the left turn direction can be effected.

When a steering wheel is rotated through a certain fixed angle in adesired direction, as described above the piston rod of the powercylinder (3) is moved in the left or right direction as a result of arelative angular displacement between the input shaft (21) and theoutput shaft (23) and thereby steering in the desired direction can beeffected. During this movement of the piston rod, since the piston rodof the power cylinder (3) is coupled to the rack (24a) as referred topreviously, the output shaft (23) is rotated via the rack (24a) and thepinion (23a) in the direction for following the rotation of the inputshaft (21) until the relative angular displacement therebetween becomeszero, that is, the torsion of the torsion bar (22) becomes zero, whenthe supply route of the working oil to the power cylinder (3) is cut offin the oil path switching valve (2), and hence the power steering systemholds a steering condition of a fixed angle. On the contrary, when thesteering wheel is returned from the steering position of the fixed angleto a neutral position, also a similar operation is effected. Such astructure and an operation of the oil path switching valve in a powersteering system has been well known in the prior art, and the details ofthe structure and operation are disclosed, for instance, in U.S. Pat.No. 3,800,407 granted to P. J. Duneah on Apr. 2, 1974.

Now, the change-over valve (11) forming oil pressure increase means willbe described in more detail. As will be apparent from FIGS. 4, 5, 7 and8(I), the change-over valve (11) is interposed midway of the bypassingoil path (7b) for the orifice (a). This change-over valve (11) includesa spool (30) having an annular groove (30a) (this annular groove (30a)forming a part of the oil path (7b)), a cap (31), a spring (33)interposed between spool (30) and cap (31), and an O-ring (34). Here itis to be noted that in FIG. 4 the spool (30) is shown at its positionoccupied upon low speed driving and when steering is effected upon highspeed driving, while in FIG. 7 it is shown at its position occupied whensteering is not effected upon high speed driving. The arrangement forthe spool (30) is such that if the oil pressure in the pilot oil path(7e₁) (See FIGS. 1 and 8(I)) rises, then the spool (30) may advanceagainst the spring (33) to open the bypassing oil path (7b), while ifthe oil pressure in the pilot oil path (7e₁) fails, then the spool (30)may retract being pushed by the spring (33) to close the bypassing oilpath (7b).

Next, the above-referred to pressure control valve (12) will beexplaihed in more detail. As will be apparent from FIGS. 5, 6 and 8(I),the pressure control valve (12) includes a sleeve (40), a spool (41), acap (42), a stop (43), a spring (44) interposed between the spool (41)and the stop (43), and a member (45) fixedly mounted within the spool(41) and having a first orifice (d). As shown in FIGS. 9, 10, 19 and 20,the spool (41) is provided with three annular grooves (41a), (41b) and(41c), and the annular groove (41a) is opposed to the oil path (7c)branched from the bypassing oil path (7b) on the upstream side of thechange-over valve (11). In addition, reference numeral (41d) designatesa chamber extending upwardly from the first orifice (d) within the spool(41), numeral (41e) designates an oil path connecting the chamber (41d)with the above-mentioned annular groove (41c) (these oil paths (41d),(41e) and (41c) forming a part of the low pressure oil path (8b)), andthe annular groove (41c) is opposed to the low pressure oil path (8b) onthe side of the valve housing (20) which oil path extends obliquelydownwards as shown in FIG. 6 from the low pressure oil path (8a) formedright above the valve body (27) of the oil path switching valve (2)shown in FIG. 2.

The above-mentioned annular groove (41a) communicates with the chamber(41d) through an orifice (e). The sleeve (40) is provided with a notch(40a) having a through-hole (40a'), a notch (40b) having a through-hole(40b'), a notch (40c) having through-holes (40c') and (40c"), a notch(40d) having a second orifice (b), and a notch (40e) having athrough-hole (40e') in succession from above to below at differentpositions in the circumferential direction on the outer peripheralsurface, as shown in FIGS. 11 to 17.

The aforementioned respective annular grooves (41a), (41b) and (41c) arearranged relative to the spool (41) in such manner that the notch (40a)having the through-hole (40a') connects the annular groove (41c) of thespool (41) with the low pressure oil path (8b) on the side of the valvehousing (20), the notch (40b) having the through-hole (40b') connectsthe annular groove (41a) of the spool (41) with the oil path (7c) on theside of the valve housing (20), the notch (40c) having the through-holes(40c') and (40c") connects the annular grooves (41a) and (41b) of thespool (41) with each other, the notch (40d) having the second orifice(b) connects the annular groove (41b) of the spool (41) with the oilpath (7e) on the side of the valve housing (20), and the notch (40e)having the through-hole (40e') connects the annular groove (41b) of thespool (41) with the oil path (7d) on the side of the valve housing (20)shown in FIGS. 3 and 5. In addition, the arrangement is such that theworking oil flowing out through the first orifice (d) into the chamber(41d) of the spool (41) returns to the oil tank (4) through the routeconsisting of the oil path (41e)→the annular groove (41c)→thethrough-hole (40a')→the notch (40a)→the low pressure oil path (8b) onthe side of the valve housing (20), that the working oil flowing fromthe bypassing oil path (7b) through the oil path (7c) into the notch(40b) flows towards the flow rate control valve (13) and the reactionpistons (5) through the route consisting of the through-hole (40b')→theannular groove (41a)→the notch (40c)→the through-hole (40c")→the annulargroove (41b)→the through-hole (40e')→the notch (40e)→the oil path (7d)in the valve housing (20), and that the working oil further flow fromthe above-mentioned notch (40c) through the oil path (7d₂) towards thereaction pistons (5). Furthermore, a part of the working oil flowingthrough the above-mentioned annular groove (41b) passes through theroute consisting of the orifice (b)→the notch (40d)→the oil path (7e) onthe side of the valve housing (20) to act upon the rear side of thespool (30) in the above-mentioned change-over valve (11) as a pilotpressure (See (7e₁) in FIG. 5), and further it flows towards the flowrate control valve (13) through the route consisting of the oil path(30b) (See FIG. 8(I)) provided at the rear end portion of the spool(30)--the oil path (7e) on the side of the valve housing (20).

Now, the above-referred flow rate control valve (13) will be explainedin greater detail. As will be apparent from FIGS. 5, 8 and 21, the flowrate control valve (13) is disposed right under the above-describedpressure control valve (12) with their axes aligned with each other. Theflow rate control valve (13) includes a sleeve (50), a spool (51), aplunger (52) made of non-magnetic material, a member (53) made ofmagnetic material that is integral with the plunger (52), a lock nut(54) for fixedly fastening the spool (51) to the plunger (52), a washer(55) butting against the sleeve (40) of the above-described pressurecontrol valve (12), a back-up spring (56) interposed between the washer(55) and the sleeve (50), an electromagnetic coil (57), a nut (58)fixedly mounted to a casing on the side of the electromagnetic coil(57), a plunger pushing force regulation bolt (59) threadedly engagedwith the nut (58), a spring (60) interposed between the bolt (59) andthe plunger (52), and a lock nut (61) for fixedly fastening the assemblyof the above-described flow rate control valve (13) to the valve housing(20). As shown in FIG. 21, the sleeve (50) is provided with an annularoil path (50a) communicating with the oil path (7d) on the side of thevalve housing (20) (See FIG. 5) and an annular oil path (50b)communicating with the oil path (7e) on the side of the valve housing(20), and in the oil path (50b) is formed an orifice (c). In addition,the above-mentioned spool (51) is provided with an annular oil path(51a) formed along the entire circumference and having a tilted groove(51a') which is formed along only a part of the circumference, and athrough-hole (51b), and the above-mentioned plunger (52) is providedwith an oil path (52a) communicating with the through-hole (51b), athrough-hole (52b) and an oil path (52c) directed in the axialdirection.

As described previously, the working oil flowing from the oil path (7d)on the side of the valve housing (20) through the oil path (7e') towardsthe flow rate control valve (13) enters the oil path (50a) in FIG. 21,while the working oil flowing through the oil path (7e) in the valvehousing (20) shown in FIG. 5 towards the flow rate control valve (13)enters the oil path (50b) in FIG. 21. FIG. 21 shows the condition ofhigh speed driving, where only the working oil entering the oil path(50b) will flow towards the member (45) on the side of the orifice (d)through the route consisting of the orifice (c)→the oil path (51a)→thethrough-hole (51b)→the oil path (52a)→the through-hole (52b)→the oilpath (52c). However, when the condition changes from a high speeddriving condition to a low speed driving condition, the spool (51)lowers, and thereby the amount of opening of the orifice (c) isdecreased, while the amount of opening of the oil path (50a ) isincreased. Eventually, under a stop condition, only the oil path (50a)is opened.

Reference symbol Q₀ in FIG. 1 represents the flow rate on the deliveryside of the oil pump (1), symbol Q₁ represents the flow rate of the oilflow entering the oil path switching valve (2) through the high pressureoil path (7a), symbol Q₂ represents the flow rate through the oil path(7c), symbol Q₃ represents the flow rate through the oil path (7e') (theoil path (50a)), symbol Q₄ represents the flow rate on the downstreamside of the orifice (c), symbol Q₅ represents the flow rate on thedownstream side of the orifice (e), and the ratio of Q₁ :Q₂ is equal toabout 6:1. In addition, the flow rate Q₂ through the oil path (7c)fulfills the equation of Q₂ =Q₃ +Q₄ +Q₅ (See FIG. 31).

The diameter of the sleeve (50) of the flow rate control valve (13) isvaried at its upper, middle and lower portions as shown in FIG. 21, thediameters being successively reduced towards the upper portion anddifferences (D₁) and (D₂) are present therebetween. On the other hand,the sleeve fitting bore on the side of the valve housing is alsoprovided so as to conform with the sleeve. Such provision is made forthe purpose of facilitating insertion of the sleeve (50) into the sleevefitting bore by reducing frictional resistance upon inserting the sleeve(50) associated with O-rings (62) into the valve housing (20), and alsofor the purpose of preventing the respective O-rings (62) from beingforced out and being pinched between the sleeve (50) and the valvehousing (20) upon insertion of the sleeve (50).

In FIGS. 22, 23 and 24 is shown a filter (70). This filter (70) consistsof a frame (71) and a wire netting (72), and is mounted by fitting atthe notch (40b) provided in the sleeve (40) of the pressure controlvalve (12) (See FIGS. 9 and 13), that is, at the inlet of the controlsystem oil path to prevent foreign matters such as dust from enteringthe control system oil path. It is to be noted that although such typeof filter could be disposed at the inlet of the high pressure oil path(7a) provided in the valve casing (20) (See the portion marked by anarrow in FIG. 4), in that case it is necessary to make the filterlarge-sized because the total delivery flow rate of the oil pump passesthrough the filter, and so, it is difficult to accommodate such alarge-sized filter by making use of the illustrated space.

It is also to be noted that the reason why the inlet of the highpressure oil path (7a) is made large in diameter is for the purpose offacilitating machining of the orifice (a) and the oil path (7b) branchedin two directions by inserting a drill through this inlet and also forthe purpose of facilitating coupling work with a piping (not shown). Inaddition, other oil paths such as (7b) [the oil path (7b) on thedownstream side of the change-over valve (11)], (7c), (7d) and (7e) arealso formed by drilling bores in the longitudinal and lateral directionsin the valve housing (20) and then plugging the bores as will be seen inFIGS. 3, 4 and 5, and in this respect also, machining of the oil pathsis facilitated. It is to be noted that reference character (Z) in FIGS.2, 3, 4, 6 and 7 designates the center axis of the oil path switchingvalve (2) and character (Z₁) in FIGS. 2 and 5 designates the center ofmeshing between the pinion (23a) and the rack (24a).

One example of the above-referred control apparatus (15) is shown inFIG. 25. Reference numeral (80) designates a constant voltage powersupply circuit, numeral (81) designates a pulse-voltage convertercircuit for sending a voltage proportional to a car speed, numeral (82)designates an error amplifier circuit, numeral (83) designates atransistor, numeral (84) designates a reset circuit which resets a timercircuit (87) at a car speed other than zero and which sets the timercircuit (87) at a zero car speed, numeral (85) designates apulse-voltage converter circuit for sending a voltage proportional to arotational speed of an engine, numeral (86) designates an enginerotational speed set circuit which sets the timer circuit (87) into astart condition when the engine rotational speed is equal to or higherthan 2000 rpm and resets the timer circuit (87) into an OFF conditionwhen the engine rotational speed is lower than 2000 rpm, numeral (88)designates a car speed input wire cut-off detector circuit which takesan ON condition upon absence of car speed pulses, numeral (89)designates a transistor, numeral (90) designates a relay, and numeral(91) designates a negative feedback circuit for stabilizing the currentflowing through the electromagnetic coil (57) of the flow rate controlvalve (13). In general, the condition where the engine rotational speedis equal to or higher than 2000 rpm at a zero car speed, cannot existnormally. Therefore, if this condition should continue 5-10 seconds ormore, then it is judged that some fault (for instance, a fault in thecar speed pulse system or a fault in a flow rate control valve system)has arisen, and feed of current to the flow rate control valve (13) (theelectromagnetic coil (57)) is interrupted by turning the relay (90) ON.

Accordingly, owing to this control circuit, current feed to the flowrate control valve (13) is interrupted upon occurrence of a fault, andhence manipulation of the steering wheel becomes heavy during high speeddriving (providing a fail-safe function), resulting in a safe operation.

Next, the operation of the above-described power steering system will beexplained. When the steering wheel is rotated from its neutral positionin the right-turn or left-turn direction and thereby the relativeangular displacement of the input shaft (21) with respect to the valvebody (27) is increased, then the output oil pressure of the oil pathswitching valve (2) (the delivery pressure of the oil pump (1)) P_(p)will rise along a quadratic curve as shown in FIG. 26. The influence ofthis delivery pressure P_(p) of the oil pump (1) appears in the oil path(7d) which is on the downstream side of the oil paths (7a), (7b) and(7c) and the pressure control valve (12) and which is on the upstreamside of the orifices (b) and (e), the flow rate control valve (13) andthe chambers (6) associated with the reaction pistons (5), and hence theoil pressure in the oil path (7d) rises in a similar manner.

The above-mentioned pressure control valve (12) controls the deliverypressure P_(p) of the oil pump (1) according to a pilot oil pressure inthe auxiliary pilot oil path (7d₁) on the downstream side of the valveitself to produce a controlled oil pressure Pc which is limited to beequal to or lower than a highest oil pressure, and also the valve (12)controls the highest pressure of the controlled oil pressure Pc as shownin FIG. 29 according to a main pilot oil pressure in the oil path (7f₁)on the downstream side of the flow rate control valve (13).

If the car is in a stopped condition, then the control apparatus (15)sends a current of i=1A (See FIG. 29) to the flow rate control valve(13) in response to a pulse signal applied from the car speed sensor(14) and thereby the plunger (52) and the spool (51) are lowered to thelower limit position (moved up to the position L in FIG. 1), so thatonly the oil path (50a) in FIG. 21 is communicated with the oil path(7f) on the upstream side of the orifice (d) via the oil paths (51a),(51b) and (52b) on the side of the spool (51) to make the oil pressurein the oil path (7f) equal to the oil pressure Pc in the oil path (7d).Under the above-mentioned stopped condition, if the steering wheelbegins to be rotated in the right-turn (or left-turn) direction, thenthe oil pressure Pc in the oil path (7d) begins to rise. Then the oilpressure in the oil path (7f) also rises in a similar manner. This oilpressure is in itself transmitted to the spool (41) (the smallerdiameter end of the spool (41)) of the pressure control valve (12) viathe main pilot oil path (7f₁), and so the spool (41) is pushed in thedirection of arrows in FIG. 10. At the same time, the working oilpassing through the annular groove (41b) of the spool (41) pushes thespool (41) in the direction of the arrows in FIG. 10 owing to adifference in the pressure acting area. On the other hand, the side ofthe spring (44) communicates with the low pressure oil path (8b), hencethe spool (41) rises successively (moves in the direction of L inFIG. 1) against the spring (44), the extent of opening of thethrough-hole (40b') decreases successively, and when the above-mentionedoil pressure pushing the spool (41) upwardly and the resilient force ofthe spring (44) balance with each other, the spool (41) will stop. Underthis condition, the maximum value of the oil pressure Pc in the oil path(7d) (in the chambers (6) associated with the reaction pistons (5))becomes lowest. If the steering wheel is further rotated in theright-turn (or left-turn) direction and the oil pressure P_(p) in theoil paths (7a), (7b) and (7c) rises further, then in the pressurecontrol valve (12) the spool (41) is moved in the direction of furtherreducing the content of opening of the through-hole (40b') owing to adifference in a pressure acting area for the oil pressure P_(p) actingupon the annular groove (41b), and so the oil pressure Pc in the oilpath (7d) is continuously maintained at the above-mentioned constant lowlevel. Accordingly, when the above-mentioned relative angulardisplacement is increased and thereby a large output oil pressure P_(p)is provided, the steering wheel torque T which is determined by the oilpressure Pc in the chambers (6) associated with the reaction pistons (5)and the torsion angle of the torsion bar (22), will not become large(See Curve (A) in FIG. 27). In the above-mentioned case of steeringunder a stopped condition, although the oil pressure Pc in the oil path(7 d) is low as described previously, since the spool (51) (See FIG. 21)is at a lowered position, the fourth orifice (c) is blocked and theworking oil will not flow through the oil path (7e). Accordingly, theoil pressure in the COV pilot oil path (7e₁) becomes the same pressureas the pressure Pc, hence owing to this pressure the change-over valve(11) opens the bypassing oil path (7b) against the resilient force ofthe spring (33), and the valve (11) is held at the position L in FIG. 1.It is to be noted that in FIG. 1 the change-over valve (11) isillustrated at its position H.

If the car is brought into a low speed driving condition, the controlapparatus (15) receives a pulse signal sent from the car speed sensorand sends a current corresponding to the car speed at each moment suchas, for instance, a current of i=0.8A to the flow rate control valve(13) to raise the plunger (52) and the spool (51) from the lower limitposition by a distance corresponding to the above-mentioned currentvalue (to move them in the rightward direction in FIG. 1) and therebydecrease the amount of opening of the oil path (50a) on the side of thesleeve (50) shown in FIG. 21. At this moment, the orifice (c) and theoil path (50b) on the side of the sleeve (50) are still kept blocked,and owing to the decrease of the amount of opening of the oil path(50a), the flow rate Q₃ passing through the orifice (d) is reduced ascompared to the flow rate passing through the oil path (50a) under theabove-mentioned stopped condition (the flow rate Q₄ being nearly zerounder this condition). It is to be noted that the amount of this flowrate decrease is absorbed by an increase of the flow rate Q₅ of the flowpassing through the orifice (e) to the low pressure oil path (8b). Sincethe flow rate Q₃ (Q₄ ≈0) of the flow coming out of the flow rate controlvalve (13) is reduced as compared to the flow rate of the flow passingthrough the oil path (50a) under the above-mentioned stopped conditionas described above, the oil pressure on the upstream side of the orifice(d) becomes lower than that under the stopped condition.

If the steering wheel begins to be rotated in the right-turn (orleft-turn) direction under the above-described low speed condition, thenthe oil pressure Pc in the oil path (7d) begins to rise. Then the mainpilot oil pressure in the oil path (7f) will also rise. This oilpressure is in itself transmitted via the main pilot oil path (7f₁) tothe spool (41) (the smaller diameter end of the spool (41)) of thepressure control valve (12), and so the spool (41) is pushed in thedirection of the arrows in FIG. 10. At the same time, the working oilpassing through the annular groove (41b) of the spool (41) pushes thespool (41) in the direction of arrows in FIG. 10 owing to a differencein a pressure acting area. On the other hand, the side of the spring(44) communicates with the low pressure oil path (8b), hence the spool(41) rises successively (moves in the direction of L in FIG. 1) againstthe spring (44), the extent of opening of the through-hole (40b')decreases successively, and when the above-mentioned oil pressurepushing the spool (41) upwardly and the resilient force of the spring(44) balance with each other, the spool (41) will stop. However, the oilpressure pushing the smaller diameter end of the spool (41) is lowerthan that under the above-described stopped condition, hence thedistance of rise of the spool is decreased by the corresponding amount(the extent of opening of the through-hole (40b') being increased by thecorresponding amount), and the oil pressure Pc in the oil path (7d) andthe chambers (6) associated with the reaction pistons (5) becomes higherthan that under the above-mentioned stopped condition. This conditionstill continues thereafter, that is, if the steering wheel is furtherrotated in the right-turn (or left-turn) direction resulting in furtherrise of the oil pressure P_(p) in the oil paths (7a), (7b) and (7c) andthe oil pressure in the annular groove (41b) tends to increase, then inthe pressure control valve (12) the spool (41) is further moved to limitthe extent of opening of the through-hole (40b'), and hence, the oilpressure Pc in the oil path (7d) is continuously maintained at aconstant level which is higher than that under the stopped condition.

Accordingly, when a large delivery pressure Pp is provided by increasingthe above-mentioned relative angular displacement, though the steeringwheel torque T becomes larger than that under the stopped condition, itdoes not become so large as that under the high speed condition as willbe described later.

If the car is brought into a high speed condition at a predeterminedspeed, then the control apparatus (15) sends a current of i=0 (See FIG.29) to the flow rate control valve (13) in response to a pulse signaltransmitted from the car speed sensor (14), to raise the plunger (52)and the spool (51) up to its upper limit position (to move them up tothe position H illustrated in FIG. 1) by means of the spring (60), andthereby only the fourth orifice (c) in FIG. 21 is communicated with theoil path (7f) on the upstream side of the first orifice (d) via the oilpaths (51a), (51b) and (52b) on the side of the spool (51). At thismoment, the fourth orifice (c) is fully opened, and while the flow rateQ4 through the fourth orifice (c) is increased, it is increased only alittle as compared to the flow rate under the above-mentioned low speedcondition. On the other hand, the flow rate Q₃ through the oil path(50a) becomes nearly zero, and therefore, the flow rate through thissystem becomes minimum. It is to be noted that this decrease of the flowrate is absorbed by further increase of the flow rate Q₅ of the oil flowthrough the third orifice (e) to the low pressure oil path (8b) (SeeFIG. 31). Since the flow rate of the oil flow coming out of the flowrate control valve (13) is reduced to the minimum as described above,the main pilot oil pressure in the oil path (7f) on the upstream side ofthe first orifice (d) becomes lowest. As this oil pressure is fed to thepressure control valve (12) via the oil path (7f₁), the highest pressureof the oil pressure Pc whose highest pressure is limited by the pressurecontrol valve (12) takes the maximum value (See FIG. 24).

If the steering wheel begins to be rotated in the right-turn (orleft-turn) direction under the above-mentioned high speed condition,then the oil pressure Pc in the oil path (7d) begins to rise. Then theoil pressure in the oil path (7f) also rises. However, since the oilpath (50a) is blocked, the amount of the pressure rise is extremelysmall. This oil pressure is in itself transmitted to the spool (41) (thesmaller diameter end of the spool (41)) of the pressure control valve(12) via the main pilot oil path (7f₁), and so the spool (41) is pushedin the direction of arrows in FIG. 10. At the same time, the working oilpassing through the annular groove (41b) of the spool (41) pushes thespool (41) in the direction of arrows in FIG. 10 owing to a differencein a pressure acting area. On the other hand, the side of the spring(44) communicates with the low pressure oil path (8b), hence the spool(41) will rise successively (move in the direction L in FIG. 1) againstthe spring (44), resulting in successive reduction of the extent ofopening of the through-hole (40b'), and when the oil pressure pushingthe spool (41) in the above-mentioned direction of the arrows balanceswith the resilient force of the spring (44), the spool (41) will stop.However, the oil pressure pushing the smaller diameter end of the spool(41) is lowest, hence the distance of rise of the spool (41) is verysmall (the extent of opening of the through-hole (40b) being large), andthe highest pressure for the oil pressure Pc in the oil path (7d) (inthe chambers (6) associated with the reaction pistons (5)) becomeshighest.

On the other hand, as the orifice (c) is opened to the oil path (51a),when the delivery pressure P_(p) itself is low, especially in the casewhere the steering wheel is in the proximity of its neutral position,the COV pilot pressure in the oil path (7e) between the orifices (b) and(c) is lowered, and this lowered pressure is transmitted to the spool(30) of the change-over valve (11) via the COV pilot oil path (7e₁),accordingly the spool (30) is lowered (the position H in FIG. 1 beingselected) to close the bypassing oil path (7b), so that the working oilsupplied from the oil pump (1) is sent to the oil path switching valve(2) via the main orifice (a) and the delivery oil pressure P_(p) israised by a preset pressure. This implies that even when steering is noteffected (the steering wheel being held at its neutral position) under ahigh speed condition, the delivery pressure P_(p) in the oil paths (7a),(7b) and (7c) rises as compared to that under a stopped condition or alow speed condition (See P_(p1) in FIG. 27). This oil pressure istransmitted to the chambers (6) associated with the reaction pistons (5)via the pressure control valve (12) and the oil paths (7d) and (7d₂),and so the reaction feeling (reaction to the hands) upon minute anglesteering under a high speed condition can be improved.

If the steering wheel is further rotated continuously in the right-turn(or left-turn) direction, the delivery pressure P_(p) in the oil paths(7a), (7b) and (7c) rises further and the oil pressure Pc in the oilpath (7d) also rises further in a similar manner to that describedabove. If the oil pressure in the oil path (7e) between the orifices (b)and (c) rises higher than a preset value and thus the force acting uponthe spool (30) via the pilot oil path (7e₁) becomes larger than theresilient force of the spring (33), then the spool (30) of thechange-over valve (11) rises (the position L in FIG. 1 being selected)to open the bypassing oil path (7b). If the steering wheel is furtherrotated continuously in the right-turn (or left-turn) condition evenafter the above-mentioned condition has been realized, then the oilpressure P_(p) in the oil paths (7a), (7b) and (7c) will rise further.However, the pressure control valve (12) controls the extent of openingof the through-hole (40b '), and hence the oil pressure Pc in the oilpath (7d) can be continuously maintained at the highest constant level.Accordingly, the steering wheel torque T for providing a large deliverypressure P_(p) by increasing the above-mentioned relative angulardisplacement becomes large (See curve (B) in FIG. 27).

As described above, the power steering system shown in FIGS. 1 to 25comprises, in a power steering system of the type in which movement of asteering wheel is transmitted via a torsion bar (22) to an oil pathswitching valve (2) to actuate a power cylinder (3) in a desiredsteering direction by switching a high pressure oil path (7a) extendingfrom an oil pump (1) to the oil path switching valve (2) and a lowpressure oil path (8a) extending from the same oil path switching valve(2) to an oil tank (4) and a part of working oil flowing through thehigh pressure oil path (7a) is led to a reaction piston (5) to restraintorsion of the torsion bar (22), parallel oil paths (7e) and (7e')branched from midway of the oil paths (7a), (7b) and (7c), a secondorifice (b) provided in one (7e) of the parallel oil paths (7e) and(7e'), a flow rate control valve (13) for discharging the working oilfed from the parallel oil paths according to a car speed so as to beproportional to the latter, a first orifice (d) for generating a mainpilot pressure depending upon a flow rate on the downstream side of thesame flow rate control valve (13), and a pressure control valve (12)actuated by the same main pilot pressure for controlling the oilpressure in the oil path (7d) extending to the above-mentioned reactionpiston (5) to be constant and to take the higher constant value as a carspeed is raised, and hence upon steering under a stopped condition theoil pressure applied to the reaction piston (5) becomes minimum.Therefore, upon steering under a stopped condition, one can drive theoil path switching valve (2) with only a small steering force (steeringwheel torque).

In addition, as the car speed rises, the oil pressure applied to thereaction piston (5) is raised. Therefore, under a high speed conditionthe oil path switching valve (2) must be driven with a relatively largesteering force, and so, under a high speed condition an appropriatereaction to the hands (feeling of a reaction force) can be obtained.

Furthermore, since the output pressure (the delivery pressure of thepump) P_(p) is led to the reaction piston (5) via the pressure controlvalve (12), the output oil pressure P_(p) presents a linearcharacteristic with respect to a steering wheel torque T within a rangeof steering under a running condition as shown by curve (B) in FIG. 27.Accordingly, the feeling of oversteering which is often encountered inthe case of the conventional power steering system is not present, andhence steering under a running condition is extremely stabilized, andsteering matched with the steering feeling can be realized.

Moreover, under a high speed condition, even if a steering wheel is in aneutral condition, as the oil pressure applied to the reaction piston(5) is raised by a predetermined value by means of the change-over valve(11), the neutral feeling of the steering wheel can be obtained under ahigh speed condition.

Furthermore, when a large degree of steering is effected by manipulatingthe steering wheel under a high speed condition, since the change-overvalve (11) is actuated to open the bypassing oil path (7b), it ispossible to make the output oil pressure P_(p) act upon the powercylinder (3) via the oil path switching valve (2) without generating areaction loss. In essence, although the pressure rise caused by theorifice (a) and closure of the change-over valve (11) is effective forimproving the feeling of reaction under a high speed condition, it willcause a pressure loss for the power cylinder (3). However, according tothe present invention, it is possible not to avoid the pressure lossupon steering under a high speed condition where an output higher than apredetermined value is necessitated for the power cylinder (3) and toreliably enhance the feeling of reaction only in the proximity of theneutral position of the steering wheel where the output oil pressure islow and thereby improve the feeling of rigidity.

Now, a description will be given in more detail with respect to theportion of the reaction pistons (5) which are most characteristic of thepresent invention. If the reaction pistons (5) are not provided aroundthe torsion bar (22) connecting the input shaft (21) with the cylinderblock (23), a neutral holding force is not present, and if an attempt ismade to increase the reaction to the hands upon running, then thesteering force under a stopped condition becomes large. However, asdescribed previously, if a plurality of reaction pistons (5) aredisposed around the input shaft (21) and the oil pressure controlleddepending upon the car speed signal is led to the chambers (6) behindthe respective reaction pistons (5), then it is possible to make thesteering force under a stopped condition small, to increase the neutralholding force and the steering reaction force during running and tothereby improve the feeling of steering. However, in the prior art (SeeFIG. 32), a plurality of reaction pistons (a) are disposed so as topinch the two wing portions of an input shaft (6) from the oppositesides. Whereas, according to the present invention (See FIGS. 3, 33 and34), a plurality of reaction pistons (5) constituted by a cylindricalmain body 501 and a protrusion 502 in the shape of a body of revolutionaround axis X--X are disposed around the input shaft (21) radially andat an equal interval in the circumferential direction. Therefore,between the respective arrangements of the reaction pistons there existsthe following difference in effects and advantages. At first, explainingthe reaction effect, a torsion torque T' in the prior art arrangementshown in FIG. 32 is represented by the following equation: ##EQU1## (anumber of reaction pistons: n, a number of effective pistons: n/2)

On the other hand, the torsion torque T in the reaction pistonarrangement according to the present invention shown in FIG. 33 isrepresented by the following equation:

T=n.p.r (a number of reaction pistons: n, a number of effective pistons:n) ##EQU2## Accordingly, the reaction effects are compared by thefollowing equation: ##EQU3##

In other words, even if the numbers of reaction pistons are the same inthe respective reaction piston arrangement, T>T' can be realized byselecting the angle θ to be small. The examples of the ratio of thetorsion torques are enumerated as follows:

    ______________________________________                                         θ = 30°                                                                      T/T' = 3.46                                                      θ = 45°                                                                       T/T' = 2.0                                                       θ = 60°                                                                       T/T' = 1.15                                                      θ = 75°                                                                       T/T' = 0.54                                                      ______________________________________                                    

Now, a description will be given on the point that the torsion torque Tcan be arbitrarily set according to the present invention. In the casewhere the input shaft (21) has been twisted by an angle φ with respectto the cylinder block (23) (See FIG. 36), assuming that thecross-section of the edges of the grooves in the input shaft (21) arelinear, the groove UVW moves to the position U'V'W' and the contactpoint between the reaction piston (5) and the groove UVW moves from theposition A to the position A'. In other words, the reaction piston (5)retracts by a distance δ and the contact point on the groove moves fromA to A'. Accordingly, the torsion torque T produced by the reactionpiston (5) can be arbitrarily set by varying the angle θ of the grooveso that the contact point between the reaction piston (5) and the groovewill come within the range of A--A" in FIG. 36, assuming that A"represents a point on line UV which fulfils the relation of A"V=A'V'. Itis to be noted that FIG. 35 shows a neutral condition of the reactionpiston (5). In addition, FIG. 37 shows another preferred embodiment ofthe groove, in which the protrusion of the reaction piston (5) is madeto butt against the central bottom portion of the groove, and FIG. 38shows still another, preferred embodiment of the groove in which theprotrusion of the reaction piston (5) is made to butt against the bothside portions of the groove. FIGS. 39, 40 and 41 show thecharacteristics of the reaction pistons (5) illustrated in FIGS. 35 and36, in FIG. 37 and in FIG. 38, respectively. FIG. 42 illustrates stillanother preferred embodiment in which the cross-section of the edges ofthe groove are made linear and on the other hand the reaction piston (5)is constituted by the cylindrical body 501 and the protrusion 502 in theform of a body of revolution about an X--X axis and having a convexlycurved cross-sectional profile and a maximum dimension transverse tosaid X--X axis which is substantially less than the diameter ofcylindrical body 501, so that the characteristic of the torsion torque Tcan be arbitrarily set, and FIG. 43 illustrates yet another preferredembodiment in which the reaction piston (5) is formed of a reactionpiston main body (5a) and a spherical body (5b) separate from and seatedin a spherical recess in main body (5a) and constituting the body ofrevolution about axis X--X.

What is claimed is:
 1. A power steering system comprising: a cylindricalinput shaft for being driven by a steering wheel, and output shaft, atorsion bar extending through the inside of said cylindrical input shaftwith one end fixedly secured to one end portion of said input shaft andthe other end projecting from the other end portion of the input shaftfor transmitting rotation of the input shaft to said output shaft, anoil path switching valve in which oil paths are switched according tothe rotational angle difference between said input shaft and said outputshaft, a power cylinder coupled to said output shaft, a high pressureoil path for supplying working oil delivered from an oil pump to saidpower cylinder via said oil path switching valve, a cylindrical portionon said output shaft surrounding the end portion of the output shaft endof said input shaft, four reaction pistons disposed in said cylindricalportion having their center lines lying in a plane perpendicular to thecenter axis of said input shaft and arrayed radially at equal intervalsin the circumferential direction of said cylindrical portion, each saidreaction pistons consisting of a cylindrical main body and a protrusionin the shape of a body of revolution around the longitudinal axis ofsaid cylindrical main body attached to the inner end of the said mainbody, said body of revolution having a convexly curved cross-sectionalprofile and a maximum dimension transverse to said longitudinal axiswhich is substantially less than the diameter of said cylindrical mainbody, a control oil path branched from said high pressure oil path andcommunicating with said reaction pistons, and a plurality oflongitudinal grooves in the outer peripheral surface of the end portionof the output shaft end of said input shaft parallel to the center axisof the input shaft and engaged by the protrusions on said reactionpistons.